It is proposed to continue and expand studies on the epidemiology of viral hepatitis being currently conducted in San Ramon-Palmares, two rural counties in Costa Rica where type A, type B and non-A/non-B hepatitis occur endemically in the absence of percutaneous mechanisms of transmission. The pattern of occurrence and the mode of spread of type B hepatitis will be studied, with emphasis on the definition of mechanisms of transmission and the role that chronic carriers play in the dissemination of the disease. The natural history of hepatitis B antigens will be studied in relation to the pathogeneis of chronic liver disease. The infective potential of the acute case and the chronic HBsAg carrier will be evaluated serologically and epidemiologically. Serologic and coprologic studies of the antigen and antibody of type A hepatitis will be done to define immunity patterns, shedding of the virus in the familial environment, spread of the infection in the community and factors leading to the occurrence of epidemics in this endemic area. Clinical, epidemiological and etio-pathogenic studies on the non-parenteral form of non-A/non-B hepatitis prevailing in the area will be performed. In addition, clinical studies will be continued on antibodies to single-stranded DNA as a diagnostic aid for the early detection of acute hepatitis and the assessment of the course of chronic hepatitis.